The present invention relates to processing techniques for articles of stainless steel and other alloys, such as, for example, tube coupling ferrules. More particularly, the invention relates to processes for case hardening such articles substantially without the formation of carbides.
As is well known, stainless steel is commonly used for many parts and assemblies. One example is a ferrule used as part of a fluid coupling for joining tube ends. The degree to which the stainless steel must be used will vary from application to application. In some high purity systems, for example in the semiconductor and biotechnology fields, lower carbon stainless steel such as 316L for example, is commonly used. Many chemistries for stainless steel are used, and other chromium bearing nickel or ferrous based alloys are known and used other than stainless steel.
One attribute of some stainless steel alloys is that thy are relatively less hard than other steel alloy materials. As a result in some applications, such as ferrules, the stainless steel article or part is provided with a hardened surface, referred to generally and herein as case hardening. The concept of case hardening is to transform a relatively thin layer of material at the surface of the part by enrichment of carbon or other ingredients to make the surface harder than the base metal alloy. This disclosure is directed to case hardening of an article by enrichment by carbon. The article thus retains in bulk the desired formability of stainless steel without the softness of the standard chemistry base metal at the article surface.
Stainless steel parts are case hardened by a process generally known as carburization. Carburization is a process by which carbon atoms are diffused in solution into the surface of the article. Known case hardening processes are performed at high temperatures. However, carburization processes performed at temperatures greater than about 1000xc2x0 F. (for stainless steel alloys) can promote the formation of carbides in the hardened surface.
It is desired, therefore, to provide new carburization processes for case hardening chromium bearing nickel or ferrous based alloy articles and that does not promote the formation of carbides.
In accordance with one embodiment of the invention, a method for case hardening a chromium bearing nickel or ferrous based alloy article includes the steps of activating the surface of the article and carburizing the activated surface at a temperature below that temperature which would promote the formation of carbides. In one embodiment the activating step is carried out by disposing a layer of iron over the surface of the article.
These and other aspects and advantages of the present invention will be apparent to those skilled in the art from the following description of the preferred embodiments in view of the accompanying drawings.